On Windows in text mode files are opened with encoding=locale.getdefaultlocale()
which is CP125X instead of UTF-8 even if $PYTHONIOENCODING=UTF-8. This
way e.g. test_strings_print fail as:

    E           Failed: not equal:
    E           Expected:
    E               print(qq(b)): "привет αβγ b"
    E               print(qq(u)): "привет αβγ u"
    E           Got:
    E               print(qq(b)): "привет αβγ b"
    E               print(qq(u)): "привет αβγ u"

where "Expected" was read from golang/testprog/golang_test_str.txt and
decoded wrongly.

-> Fix it by always opening files for reading in binary mode and
utf8-decoding manually, if needed, everywhere.